1. Field of the Invention
This invention relates generally to a speed change lever assembly which is used for displacing a front or rear deraileur relative to a multiple sprocket. More particularly, the present invention concerns a speed change lever assembly of the type wherein pressing of one lever causes the deraileur to move in one direction, whereas pressing of another lever causes the deraileur to move in the opposite direction.
2. Description of the Prior Art
As is well known, a bicycle speed change system comprises a deraileur mounted adjacent a front or rear gear (multiple sprocket) for movement axially of the gear, and a lever assembly mounted on a suitable portion of a bicycle frame or handlebar and connected to the deraileur by means of a control cable. The lever assembly is operated to displace the deraileur relative to the gear, thereby shifting the chain from one sprocket to another of the gear for speed change.
A typical speed change lever assembly includes a single lever pivotally supported at its boss portion on a fixed shaft. The boss portion of the lever works also as a cable winder connected to one end of a control cable whose other end is connected to a deraileur. When the lever is pivoted in one direction, the control cable is wound up on the lever boss portion against a tension applied to the cable by a return spring which is incorporated in the deraileur. When the lever is pivoted in the opposite direction, the cable is paid out from the lever boss portion under the tension of the return spring. Such cable movements are transmitted to the deraileur for movement thereof relative to a front or rear gear, thereby performing an intended chain shift.
With the lever assembly described above, the lever has to be pressed and pulled respectively for winding up and paying out the cable. Therefore, it is necessary for the cyclist to use two different kinds of finger movements for operating the lever in the two pivotal directions. Thus, the operability of the typical lever assembly is relatively poor. Such poor operability becomes particularly problematic when the lever provides a large pivotal angle.
In an attempt to solve the above problem, there has been proposed to use a double lever assembly, as disclosed for example in Japanese Patent Publication No. 55-20909 (Published: June 5, 1980; Applicant: Bridgestone Cycle Co., Ltd.; Inventor: Kunitoshi KOSAKAI) or Japanese Utility Model Publication No. 59-43191 (Published: Dec. 20, 1984; Applicant: Shimano Industrial Co., Ltd.; Inventor: Keizo SHIMANO). Specifically, the double lever assembly comprises a pair of levers pivotally supported on a common pivot shaft respectively on both sides of a fixed plate. The fixed plate has a retaining hole for holding a ball whose diameter is slightly larger than the wall thickness of the fixed plate, so that the ball is partially projectable from the retaining hole on either side of the plate. One lever (first lever) is connected to a control cable under a tension, and formed with a positioning hole which disengages from the ball in its first pivotal limit position (cable pay-out position) but engages with the ball in its second pivotal limit position (cable winding position). The other lever (second lever) is also formed with a positioning hole which engages with the ball in its first pivotal limit position but disengages from the ball in its second pivotal limit position. Both levers are always urged toward their respective first pivotal limit positions by the tension of the control cable and/or a separate spring.
In operation, when the first lever assumes its first limit position with the positioning hole thereof disengaging from the ball, the second lever is held in its first limit position by the ball engaging with the positioning hole thereof. In this condition, the control cable is fully paid out from the first lever. When the first lever is pivoted to its second limit position to fully wind up the control cable, the ball comes into engagement with the positioning hole of the first lever but disengages from the positioning hole of the second lever, so that the second lever is pivoted by the urging force of the spring to its second limit position which substantially corresponds to the first limit position of the first lever. On the other hand, when the second lever is pressed to its first limit position, the ball comes into engagement with the positioning hole of the second lever lever but disengages from the positioning hole of the first lever, so that the first lever is pivoted to its first limit position by the tension of the control cable.
In this way, the double lever assembly is capable of shifting up and down only by pressing either one of the two levers. However, such a lever assembly is disadvantageous in the following respects.
First, each of the levers can take only two limit positions, so that the prior art double lever assembly is not capable of selecting three or more speeds. Second, the pivotal movement (returning movement) of the first lever from its second position to its first position is initiated only after the second lever has been completely pivoted to its first position. Thus, the pivotal pressing operation of the second lever does not cause the control cable to be immediately paid out from the first lever. In other words, the prior art double lever assembly fails to provide a quick operating response at least with respect to the cable pay-out operation.
Japanese Utility Model Application Laid-open No. 60-149485 (Laid-open: Oct. 4, 1985; Applicant: Maeda Industries, Ltd.; Inventor: Nobuo OZAKI) discloses an improved lever assembly which comprises a first pivotal lever, a second pivotal lever, and an indexing mechanism provided between the first and second levers. The first lever is connected to one end of a control cable and pivotally urged in a cable pay-out direction by a tension applied to the control cable as well as by a spring. The second lever is urged by the above-mentioned spring in the same pivotal direction as the first lever.
The indexing mechanism of OZAKI includes an engaging member carried by the second lever, and an indexing track carried by the first lever and held in contact with the engaging member by the urging force of the spring. The indexing track has a series of stopper steps which successively come into stopping engagement with the engaging member when the first lever is pivoted in a cable winding direction against the cable tension. The engaging member is disengageable from the stopper steps to allow pivotal movement of the first lever in the cable pay-out direction when the second lever is pivoted against the urging force of the spring.
Thus, the OZAKI's lever assembly is capable of providing any desired number of selectable speeds by suitably selecting the number of the stopper steps. However, the OZAKI's lever assembly is disadvantageous in the following point.
In the OZAKI's lever assembly, the first lever is always urged in the cable pay-out direction by the cable tension and the spring force, and only the engagement of the engaging member with the stopper steps of the indexing track prevents the first lever from pivoting in the cable pay-out direction. Thus, if the second lever is improperly pressed to such an extent that the engaging member is brought too much away from the indexing track (the stopper steps), the first lever having been once pivoted maximally in the cable winding direction will be pivoted fully in the cable pay-out direction by skipping the interim stopper steps, consequently failing to achieve an intended speed change. Therefore, it is necessary to carefully operate the second lever in order to ensure that the first lever is pivoted stepwise in the cable pay-out direction.